1. Field of the Invention
The invention relates to the field of voltage controlled lowpass filters, particularly those employing MOS devices.
2. Prior Art
Numerous active and passive lowpass filters are very well-known in the art. Perhaps one of the most commonly used active filters in the audio range, employs an integrated circuit amplifier such as an operational amplifier with external feedback. This filter is very versatile and has excellent filtering characteristics, however, in many applications it is too large and too expensive, particularly since external components are required. In some applications there is a need for a lowpass filter which may be readily fabricated as part of an integrated circuit without external components. Often the more precise filtering characteristics which may be obtained with an active filter employing an operational amplifier are not required.
Frequency considerations in metal-oxide-semicondcutor (MOS) devices for the most part have been confined to increasing the cutoff frequency or maximum operating frequency of field-effect transistors. To this end, higher mobility substrate materials have been employed. Self-aligned gate structures have permitted reduction in parasitic gate-to-drain and gate-to-source capacitance further increasing frequency performance. In some cases, smaller channel lengths are employed to obtain higher transconductance and lower active channel capacitance. The frequency characteristics of MOS devices are considered only as a limitation on performance, thus the focus of attention has been on increasing the cutoff frequency or maximum operating frequency.
In this application, an MOS device is described which has the general characteristics of a voltage controlled lowpass filter in the audio range. The MOS device may be readily fabricated as part of an integrated circuit with well-known MOS fabrication techniques.